Syphilis remains a public health threat worldwide, with an estimated 12 million new infections per year and a global burden of 25 million infections. Within the last decade there has been an increase in syphilis outbreaks in major cities around the world, with a 10-fold increase in syphilis infections documented for China and Canada and a doubling of syphilis infections in the United States. Syphilis infections increase the risk of acquiring and transmitting HIV, and there has been a 23% increase in congenital syphilis infections, resulting from mother to child transmission in utero, in recent years within the United States. Although syphilis is curable with penicillin treatment if diagnosed early, the worldwide syphilis prevalence shows that elimination of this disease will not occur through public health control measures alone, and instead will require development of an effective syphilis vaccine. Development of a syphilis vaccine requires an in depth knowledge of the pathogenic mechanisms used by this highly successful pathogen. The bacterium that causes syphilis, Treponema pallidum, is able to disseminate rapidly within the host during the early stages of infection to infect every organ and tissue. The pathogenic mechanisms used by T. pallidum to undergo widespread dissemination throughout the host are not known, and gaining understanding within this highly relevant area of study will reveal novel vaccine candidates that can be targeted to prevent establishment of infection. The long-term objective of the research project is to elucidate the mechanisms that facilitate attachment of T. pallidum to host components and widespread treponemal dissemination and, specifically, to determine the role of two T. pallidum proteases, pallilysin and Tp0750, in this important pathogenic process. To accomplish this objective, the following specific aims are proposed: (1) to elucidate the mechanism of host component attachment and proteolysis for pallilysin and Tp0750; (2) to determine the ability of these two proteases to exploit the host fibrinolytic process that is essential for normal host component degradation and turnover; (3) to use sensitive proteomic methodologies and a model treponeme to determine the location of these two proteases within T. pallidum; and (4) to directly determine the role these two proteases play in dissemination of T. pallidum. These studies will increase understanding of the critical process of T. pallidum dissemination and will reveal suitable vaccine candidates for prevention of syphilis infection.